1. Technical Field
The present disclosure relates to a composite transformer which is provided with a plurality of transformer units and has functions of power transmission and voltage conversion between primary and secondary and also a function of signal transmission between primary and secondary.
2. Background Art
Patent Document 1, Japanese Unexamined Patent Application Publication No. 2000-260639, discloses an arrangement in which a plurality of transformer units which can be regarded as being independent of each other are formed using a pair of cores.
FIGS. 1(A)-1(D) and FIG. 2 are diagrams illustrating an example of a configuration of a transformer according to Patent Document 1. FIGS. 1(A)-1(D) are diagrams illustrating patterns of individual layers of a multilayer substrate.
FIG. 2 is a cross-sectional side view of the transformer which is taken along lines X1-X1, X2-X2, X3-X3, and X4-X4 shown in FIG. 1. As illustrated in FIGS. 1(A)-1(D) and FIG. 2, this transformer has a multilayer substrate composed of eight layers, including a first printed coil substrate 11, a second printed coil substrate 12, a third printed coil substrate 13, a fourth printed coil substrate 14, and layers forming return wires of these coils.
In this eight-layer multilayer substrate, two outer holes 11a and 11c and one middle hole 11b, through which two outer legs 6a and 6c and one middle leg 6b of an E-E-shaped or E-I-shaped three-leg core pass, are arrayed in a straight line. A pair of cores of the three-leg core are fitted to each other with the multilayer substrate sandwiched between, so that closed magnetic paths are formed.
In the three-leg core, the two outer legs have the same cross-sectional area, and the distances between the middle leg and the individual outer legs are equal. Two magnetic paths formed by the middle leg and the individual outer legs have the same magnetic resistance. On each of the first printed coil substrate 11 and the third printed coil substrate 13, a coil is spirally wound around the middle hole 11b, so that a primary coil and a secondary coil of a first transformer unit is formed. In each of the second printed coil substrate 12 and the fourth printed coil substrate 14, a clockwise coil is wound around one of the outer holes and a counterclockwise coil is wound around the other one of the outer holes with the same number of turns. These coils are connected in series to form a primary coil and a secondary coil of a second transformer unit.
In the second transformer unit, a voltage induced in the coil wound around the one of the outer holes by the action of the first transformer unit is canceled so as to be zero volts by a voltage induced in the coil wound around the other one of the outer holes, which is applied in the opposite direction and is equal in absolute value to the voltage induced in the coil wound around the one of the outer holes. A magnetic flux generated in the one of the outer legs and a magnetic flux generated in the other one of the outer legs by the action of the second transformer unit cancel out each other in the middle leg. Thus, the action of the second transformer unit does not affect the first transformer unit. Utilizing this principle, the first transformer unit and the second transformer unit operate independently of each other while sharing one core.
Improvements to be Made
In Patent Document 1, the outer legs having coils wound therearound with the same number of turns in mutually opposite directions are arranged at opposite positions via the middle leg and have a distance therebetween. Such an arrangement can be further improved, as follows.
First, there exists the possibility of occurrence of a failure in which interference occurs between individual transformer units due to a manufacturing problem. The first and second transformer units in Patent Document 1 can independently operate when a pair of cores are properly fitted to each other. However, when a small gap exists at the joint of either one of the outer legs due to dust or the like, interference due to the two legs occurs, resulting in malfunction of an electronic device using the transformer units, depending on the degree of the interference. A greater amount of magnetic flux, which has passed along the middle leg, passes along a magnetic path that has smaller magnetic resistance than the other parallel magnetic path. The magnetic path formed by the outer leg having a small gap and the middle leg has a larger magnetic resistance than the other magnetic path formed by the other outer leg and the middle leg, and thus has a smaller amount of magnetic flux passing therealong than the other magnetic path. Therefore, the voltage induced in the coil wound around the outer leg with the small gap is smaller than that induced in the coil wound around the other outer leg. Accordingly, the voltage generated by the action of the first transformer unit is not completely canceled, and the voltage appears at the output of the second transformer unit. On the other hand, also regarding a magnetic flux generated by the action of the second transfer unit, a larger magnetic flux is present in the other outer leg than the outer leg with the small gap. Thus, the magnetic fluxes do not completely cancel out each other in the middle leg, and the voltage generated by the second transformer unit appears at the output of the first transformer unit.
Second, a large area is necessary for pattern wiring. In order to serially connect coils wound around outer holes with the same number of turns in mutually opposite directions, it is necessary to provide two wires for go and return paths between the individual coils. However, when the distance between the two outer legs is large, the length of the two wires is large, which leads to a large area being necessary for pattern wiring. When pattern wiring of the coils of the first transformer unit and the second transformer unit is provided on the same layer in a multilayer substrate, the area necessary for the pattern wiring of the coils wound around the outer holes increases. As a result, the pattern area for the coils of the first transformer unit has to be decreased by the increase of the area necessary for the pattern wiring for the coils. Consequently, the thickness of the coil of the first transformer unit decreases and the resistance of the coil increases. When the first transformer unit receives a high power, loss is increased due to an increase in the resistance of the coil.
Third, an increase in the distance between the outer legs increases the length of a magnetic path formed by the cores in the second transformer unit, and thus an AL value (inductance per one turn) decreases. When a pulse signal is to be transmitted by the second transformer unit, a reactive current according to an exciting inductance flows. Thus, loss increases when the exciting inductance decreases.
When the number of turns of the coil in the second transformer unit is increased in order to compensate for the decrease in the exciting inductance, the area necessary for pattern wiring increases. Further, when the number of turns of the coil in the second transformer unit is increased while the second transformer unit interferes with the first transformer unit, the interference voltage increases with increasing number of turns. This leads to malfunction of an electronic apparatus using such a transformer.